1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium used in, e.g., a hard disk drive using the magnetic recording technique, and a magnetic recording/reproduction apparatus using the same.
2. Description of the Related Art
While large-capacity hard disk drives have been desired in recent years, the increase in medium noise is becoming a problem as the recording density increases. The main cause of the medium noise is presumably a zigzagged magnetic domain wall in a bit boundary. That is, the bit shape is determined in accordance with various factors such as the writing magnetic field of a head and the size of magnetic crystal grains forming the medium. The medium noise is produced particularly because the formation position of the bit boundary is indefinite due to variations in grain size. To reduce the noise, the unevenness of the recording bit boundary must be made as small as possible. To reduce the unevenness of the recording bit boundary, it is possible to downsize magnetic crystal grains forming the magnetic recording layer.
If downsizing of the magnetic crystal grains advances, however, the thermal decay resistance of the magnetic recording layer decreases at the same time. To reduce the unevenness of the recording bit boundary while the thermal decay resistance of the magnetic crystal grains is maintained, it is effective to make the grain diameter distribution uniform. However, it is difficult to make the grain diameter distribution uniform while the grain diameter of the crystal grains is maintained at about 10 nm or less as the present level.
Also, if a material which forms a grain boundary region for dividing the magnetic crystal grains is added to the magnetic recording layer in order to downsize the magnetic crystal grains, the alignment of the magnetic crystal grains worsens by diffusion of the material which forms the grain boundary region. To improve the alignment of the magnetic crystal grains, therefore, it is necessary to increase the film thickness of the underlayer or magnetic recording layer. Especially in a double-layered perpendicular magnetic recording medium, this increases the distance between a magnetic recording head and a soft magnetic backing layer for refluxing the head magnetic field, and weakens the effective magnetic field from the magnetic head (produces a spacing loss), thereby worsening the recording/reproduction characteristics of the perpendicular magnetic recording medium. Accordingly, to make the magnetic crystal grains small and uniform, it is necessary to decrease the film thickness of the magnetic recording layer and improve the crystallinity of the magnetic crystal grains.
As a technique of obtaining a fine uniform film in the fields of quantum electronic devices such as single-electron transistors and single-electron memories, a technique which makes a fine uniform nanostructure from Al and Si is disclosed. In this technique, a regular region for preferentially growing Al is formed on a substrate, and a mixed film mainly containing Al and Si and/or Ge is formed after that. The total amount of Si and/or Ge contained in this mixed film is 20 to 70 at %. This makes it possible to form a mixed film having a plurality of cylinders mainly containing Al, having a diameter of 1 to 30 nm, and spaced at intervals of 30 nm or less, and a matrix region mainly containing Si and/or Ge and surrounding these cylinders, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2004-193523. Unfortunately, to form the regular region, this technique requires the formation of micro-recesses of a honeycomb array using a wet process based on the technique of anodic oxidation and focused ion beam (FIB). These wet process and FIB are not easily applicable to the magnetic recording medium manufacturing process which requires a high manufacturing cost and in which film formation is mainly performed in a vacuum. In addition, Si contained in the AlSi film readily diffuses. Therefore, if this AlSi film is directly introduced to the magnetic recording medium, adjacent layers are adversely affected. Furthermore, it is difficult to maintain good crystal alignment by a small film thickness. For these reasons, the above-mentioned nanostructure cannot be directly applied to the field of magnetic recording.
Also, another technique for a perpendicular magnetic recording medium having a soft magnetic backing layer, alignment control layer, grain diameter control layer, underlayer, and perpendicular magnetic recording layer on a substrate is disclosed, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2004-30767. In this technique, the grain diameter control layer mainly contains at least one element selected from the group consisting of silver, aluminum, tantalum, copper, and gadolinium. This makes it possible to control the alignment and grain diameter of magnetic crystal grains of this grain diameter control layer, and increase the thermal decay resistance, thereby increasing the S/N and resolution. However, this technique is still unsatisfactory to achieve a high recording density exceeding, e.g., 100 Gbits/inch2. Accordingly, it is being desired to further downsize and uniformize the magnetic crystal grains.